Cable transmission systems are widely used to transmit carrier signals in the communications field. Typically, such a system utilizes a multipaired cable having repeater amplifiers spaced at periodic intervals along the system. These amplifiers are often line powered amplifiers wherein a constant current, which is fed longitudinally down one cable pair and back along another from a main terminal, is used to power the repeater amplifiers connected thereto. Each of the amplifiers must be protected from hazardous voltages which originate from three primary sources. (1) Lightning strikes--generally caused when a bolt of lightning elevates the local ground potential which in turn causes a large potential gradient between the grounded cable sheath and the cable core resulting in arc-over to one or more cable pairs thereby causing high voltages to strike one or more of the amplifiers. (2) Induced power line surges--generally result when the telephone transmission cables and the power line cables are carried on parallel (e.g. the same) poles. These surges may be continuous where there is an imbalance on the power transmission system, or momentary when a break, arc-over or short occurs in the power system. (3) Power line conduction--generally results when a break in the power cable causes a conductor to fall across the telephone cable causing direct conduction through the cable sheath to ground. This in turn elevates the potential of a localized area of the cable sheath relative to its core.
Various protection schemes have been used in the past, generally involving the use of negative impedance switching devices such as carbon protector blocks or gas tubes which have been connected in a variety of configurations across the repeater amplifier terminals. These devices present a very high impedance to the circuit until their breakdown voltage is reached whereupon they present a relatively low impedance until the hazardous voltage is removed. With the introduction of data communication systems utilizing higher carrier frequencies than the earlier analog systems, gas tubes are generally preferred to carbon blocks as they do not suffer from dust and moisture which can cause leakage and noise problems. However, despite the use of gas tubes, problems protecting the system have remained. It has been found that one reason for this is that the requirements for protecting the repeater amplifiers against lightning strikes are somewhat different from those required to protect them against power line surges.
In the past, negative impedance switching devices have been connected across both the input and the output terminals of the repeater amplifiers as well as between both their input and output terminals. At least one such device is also connected between the terminals of each amplifier and a common bus at each of the repeater sites. In some applications, these buses have been allowed to float thereby protecting the circuitry against relatively low voltage power line surges. However, this arrangement does not provide satisfactory protection against lightning strikes where exceedingly high voltages are introduced between the grounded cable sheath and the cable core resulting in uncontrolled arc-over within the system. In still other applications the common bus has been grounded. With this arrangement, the uncontrolled breakdown is minimized. However, the surge currents are now coupled through specific components, often resulting in their destruction.